Current vectors for gene delivery and gene therapy are comprised of viral based and non-viral based systems. Lipid-based non-viral systems include cationic lipid plasmid DNA complexes. Limitations of these systems include large sizes, toxicity and instability of the complexes in the serum. Unfortunately, the foregoing drawbacks limit the applications for these complexes.
Researchers have devoted tremendous effort to the design of long circulation stealth liposomes that can be used for systemic delivery (see, Papahadjopoulos, D. et al., Proc. Natl. Acad. Sci. 88:11460-11464 (1991); Klibanov, A. L. et al., J. Liposome Res., 2:321 (1992); Woodle, M. C. et al., Biochim. Biophys. Acta., 1113:171(1992); Torchilin, V. P. et al., In: Stealth Liposomes. Ed. By D. Lasic, F. Martin. CRC Press, Boca Raton, Fla., pp. 51-62 (1995); Allen, T. M. et al., Biochim. Biophys. Acta., 1237:99-108 (1995) and Zalipsky, S. et al., J. Controlled Release, 39:153-161 (1996)). In certain instances, and depending on the formulation, stealth liposomes are often comparatively inefficient at facilitating cellular uptake and therefore the therapeutic efficacy is reduced.
In general, the molecular mechanism of liposomal longevity in vivo can be attributed to steric hindrance resulting from hydrophilic polymer surface barriers. The hydrophilic polymer barriers prevent or reduce the rate of the adsorption of macromolecules from the blood and sterically inhibit both electrostatic and hydrophobic interactions between liposomes and blood components. Thus, although the longevity of stealth liposomes has been increased by the insertion of hydrophilic polymers, the cellular uptake of the stealth liposomes often is inefficient.
Over the past decade, it has also become clear that liposomal systems possessing cationic lipids are highly effective transfection agents in vitro (Felgner, P. L. et al., Nature 337:387-388 (1989); Felgner, P. L. et al., Proceedings of the National Academy of Sciences of the United States of America 84:7413-7417 (1987)). The addition of cationic liposomes to plasmid DNA gives rise to large DNA-lipid complexes that possess excellent transfection properties in vitro, but which are ineffective in vivo due to their rapid clearance from the circulation by cells of the reticuloendothelial system (RES). The need for a non-viral lipid-based system capable of systemic delivery of genes to cells led to the recent development of stabilized plasmid-lipid particles (SPLPs) (Wheeler, J. J. et al., Gene Therapy 6:271-281 (1999)). These particles are small (about 70 nm), contain a single copy of a plasmid vector, possess stealth properties resulting from a surface coating of poly(ethyleneglycol) (PEG), and protect DNA from degradation by serum nucleases.
Enhancing intracellular delivery of liposomes and/or their contents represents one of the major remaining problems in the development of the next generation of drug delivery systems. In order to optimize the delivery of drugs (conventional or genetic) in vivo, general methods for increasing the interactions of liposomes with cells need to be developed. To date, attempts include the use of specific targeting information on the liposome surface, such as an antibody (see, Meyer, O. et al., Journal of Biological Chemistry 273:15621-15627 (1998); Kao, G. Y. et al., Cancer Gene Therapy 3:250-256 (1996); Hansen, C. B. et al., Biochimica et Biophysica Acta 1239: 133-144 (1995)), vitamin- (see, Gabizon, A. et al., Bioconjugate Chemistry 10:289-298 (1999); Lee, R. J. et al., Journal of Biological Chemistry 269:3198-3204 (1994); Reddy, J. A. et al., Critical Reviews in Therapeutic Drug Carrier Systems 15:587-627 (1998); Holladay, S. R. et al., Biochimica et Biophysica Acta 1426:195-204 (1999); Wang, S. et al., Journal of Controlled Release 53:39-48 (1998)), oligopeptide- (see, Zalipsky, S. et al., Bioconjugate Chemistry 6:705-708 (1995); Zalipsky, S. et al., Bioconjugate Chemistry 8.111-118 (1997)), or the use of oligosaccharide constructs specific for a particular membrane protein or receptor. Unfortunately, these methods have not been successful in achieving this goal, despite promising in vitro results. While specific targeting of liposomes to tissues remains an important area of research, other approaches may also provide significant improvements in the effectiveness of liposomal carriers.
In view of the foregoing, what is needed in the art is a lipid-based drug formulation with increased longevity coupled with increased cellular uptake. The present invention satisfies this and other needs.